A system is already known for measuring the angle of incidence of an aircraft, for example an aeroplane or a helicopter, the incidence being the angle formed between the longitudinal axis of the aircraft and the airflow, i.e. the air speed vector, which system is also known as incidence probe, which comprises a CO.sub.2 laser generator with monochromatic spectral line, an optical system for emitting the laser beam, an optical system for receiving the back-scattered beam, which is also monochromatic and comes essentially from the aerosols whose movement or agitation is caused by that of the air and an interferometer receiving a part of the emitted beam and the back-scattered beam, and producing a transposition from the field of optical waves to the field of radiofrequency waves, so as to derive therefrom the Doppler back-scatter frequency and so the component of the air speed along the axis of the emitted laser beam.
Such a system is a longitudinal anemometry system.
The air speed and the incidence are parameters which may be displayed on the instrument panel of the aircraft and which serve for controlling and piloting the aircraft, for elaborating then displaying certain flight parameters of the aircraft.
But such an anemometric system which might be carried on board an aircraft would have drawbacks. A CO.sub.2 laser generator, because of its cooling device, is expensive and cumbersome. The space which would moreover be necessary in order to use a CO.sub.2 laser generator in such an application would be further increased by the necessary proximity of the interferometer and the pumping means of the laser generator. Since, in the application considered, namely an anemometric measurement, the laser generator would have to be located close to the "skin" of the aircraft, it can be readily seen that the CO.sub.2 laser generator is finally not very appropriate for such an application, at least as mass produced equipment.
Moreover, there exists fringe anemometry and passage time anemometry which allow measurements to be made, not along the axis of the beam, but transversely to the beam and at short distances. Now, measurements at a short distance from an aircraft are subject to errors due to the aerodynamic disturbances.